It is not like we haven't seen this syndrome in royals and celebrities repeatedly.
Princess Margaret was similarly disposed to cutting loose, but it was a day in which candid photography would likely have just been censored.
And, more to the point, it just looks like harmless fun.
As it is, if showing your ass is a problem for Buckingham Palace, they've had to deal with it with Phillip, Charles, Diana, Fergy. . . just with less photographic evidence usually.